As a means for increasing the heat shielding effect of a window of a building, a heat-ray absorbing glass is used. For example, Patent Literature 1 and Patent Literature 2 each disclose a heat-ray absorbing glass containing a specific combination of metal oxides. There have recently been developed double glazed glass panels having not only heat shielding properties but also heat insulating properties. For example, Patent Literature 3 discloses a double glazed glass panel including a heat-ray absorbing colored glass sheet provided on the outdoor side, a clear glass sheet provided on the indoor side, and a low emissivity film formed on the indoor-side surface of the outdoor-side heat-ray absorbing colored glass sheet.
In a conventional double glazed glass panel, an air layer (or a gas layer such as an argon gas layer) between an outside glass sheet and an inside glass sheet need have a thickness of at least 6 mm to ensure the heat insulating properties, and thus the entire double glazed glass panel has a large thickness. For example, in the case where the outside glass sheet and the inside glass sheet each have a thickness of 3 mm and the air layer has a thickness of 6 mm, the entire double glazed glass panel has a large thickness of 12 mm and is not suitable for use in window frames of residential houses. Under these circumstances, there has been developed a reduced pressure double glazed glass panel including a very thin vacuum layer with a thickness of about 0.2 mm provided between outside and inside glass sheets, instead of an air layer between the glass sheets.
If a heat-ray absorbing colored glass sheet is provided on the outdoor side and a low emissivity film is formed on the indoor-side surface of this heat-ray absorbing colored glass sheet in such a reduced pressure double glazed glass panel, the glass panel can, in spite of its small thickness, reduce the intensity of solar radiation coming into a room during the summer daytime and block incoming heat from outside during the summer nighttime. This reduced pressure double glazed glass panel can not only increase the cooling efficiency in summer but also increase the heating efficiency in winter.
In a conventional, common double glazed glass panel, a soft material is used as a sealing material. In contrast, in the reduced pressure double glazed glass panel, a hard material such as a low melting glass or a metal solder is used as a sealing material. Therefore, when the reduced pressure double glazed glass panel is exposed to solar radiation and the temperature difference between the outside glass sheet and the inside glass sheet increases, the difference in thermal expansion between the glass sheets cannot be absorbed due to the deformation of the sealing material, which may cause bending of the glass sheets. In particular, when the rigidity of the window frame is not sufficient, the frame is also bent subsequent to the bending of the glass sheets. When the frame is bent, not only the operation of opening and closing the window cannot be performed smoothly, but also the frame and the panel may scrape against each other and be damaged. Patent Literature 4 discloses a reduced pressure double glazed glass panel which is less susceptible to bending when exposed to solar radiation.